1. Field of the Invention
The invention relates to a voltage adjusting apparatus and, more particularly, to a voltage adjusting apparatus for adjusting an output voltage of a power adaptor according to a usage state.
2. Description of the Prior Art
A power adaptor is generally composed of a transformer and a rectifier, and it can adjust a voltage to be high or low and output either a direct current (DC) or an alternating current (AC). Some power adaptors do not include a transformer. For a general larger electrical appliance such as a stereo, the power adaptor is hidden in the electrical appliance and mostly is disposed at the place where a power supply is connected. For example, the power adaptor of a computer is disposed at the place where a computer power line is connected. However, in a few portable electrical appliances such as mobile phones, notebook computers, and walkmans, based on the consideration of the volume of the electrical appliance, the power adaptor cannot be placed in the electrical appliance. Therefore, most of the portable electrical appliances use external power adaptors.
Please refer to FIG. 1A. FIG. 1A is a schematic diagram showing a power adaptor 1 converting an AC to a DC according to the prior art. Generally, a conventional power adaptor 1 converts an AC to a DC via a voltage adjusting technique, as shown in FIG. 1A. At that moment, an output voltage is a constant value. For example, the output voltage of a notebook computer is generally 19 V.
FIG. 1B is a schematic diagram showing a circuit of the power adaptor 1 in FIG. 1A. As shown in FIG. 1B, the power adaptor 1 includes three resistors 10, 12 and 14 and a voltage regulator 16. If the output voltage needs to maintain 19 V, the resistance values of the three resistors 10, 12 and 14 should be 16.5 KΩ, 10 KΩ, and 3.3 KΩ, respectively, and the voltage regulator 16 needs to regulate the voltage of a node N1 to maintain 2.5 V. That is, the output voltage=2.5/(3.3K//10K)*16.5K+2.5=19.1.
However, the output voltage needed by a notebook computer in an idle state is usually lower than the output voltage needed in a usage state. If the output voltage is not adjusted with the usage state, the notebook computer may still cause more power supply losses in the idle state, which increases system temperature and reduces the reliability of electronic components. Furthermore, if the system temperature increases, correspondingly, the rotation speed of a fan also needs to increase to accelerate heat dissipation. Once the rotation speed of the fan increases, noise increases with the rotation speed, which makes trouble for users.
Generally speaking, the power supply losses include conduction losses and switching losses.
The conduction losses are the resistance losses in an appliance due to current flowing in the on-state resistance (Rdson) of the mental-oxide-semiconductor field-effect transistor (MOSFET). The conduction losses may be calculated from the following two equations.PCHS=Iout2*Rdson*D. PCLS=Iout2*Rdson*(1−D).
PCHS represents the conduction losses of a high side of the MOSFET. PCLS represents the conduction losses of a low side of the MOSFET. D represents a conduction ratio (D=Vout/Vin), where Vout represents an output voltage, and Vin represents an input voltage of a power supply. Tout represents a load current. Rdson represents an on-state resistance of the MOSFET. Since D and Iout are determined by a practical application, Rdson should be chosen as low as possible.
The switching losses are the losses due to switching the high side and the low side of a MOSFET. The switching losses may be calculated from the following two equations.
            P      ⁢                          ⁢      D      ⁢                          ⁢      H      ⁢                          ⁢      S        =                            (                                    t              r                        +                          t              f                                )                *                  V          in                *                  I          out                *                  f          s                    2                  P      ⁢                          ⁢      D      ⁢                          ⁢      L      ⁢                          ⁢      S        =                            (                                    t              r                        +                          t              f                                )                *                  V          d                *                  I          out                *                  f          s                    2      
PDHS represents the switching losses of a high side of the MOSFET. PDLS represents the switching losses of a low side of the MOSFET. tr represents rise time, and tf represents fall time. fs represents a DC-DC converter switching frequency. Vd represents an on-state voltage of a body diode. The rest of the parameters are the same with the above parameters.
When Iout=15 A, Vd=1.5 V, the high side Rdson=9 mΩ, the low side Rdson=5 mΩ, tr+tf=80 ns, and fs=200 kHz, the losses are calculated as shown in table 1.
TABLE 1output voltage (V)1516171819PCHS + PDHS2.0032.112.2192.3292.44PCLS + PDLS1.1931.21.2061.2111.216total losses (W)3.1963.313.4253.54 3.656
According to table 1, in the same situation, if only the output voltage is reduced, losses are reduced by at most about 15˜20% to efficiently achieve an energy-saving effect.